Conventional lighting apparatuses using light-emitting diodes (called “LEDs” from now on) include the following:
As a first conventional example, an apparatus is known which has a plurality of LEDs connected in series, supplies the LEDs connected in series with a voltage boosted from a power supply voltage by a DC/DC converter, and causes the same current to flow through all the LEDs to light them (see Patent Document 1, for example).
As a second conventional example, an apparatus is known which employs a charge pump type boosting power supply using a capacitor, and arranges a constant current circuit using a transistor in series with an LED element to carry out constant current drive of the LED element (see Patent Document 2, for example).
As other conventional technologies relating to lighting of LEDs, the following Patent Document 3 to Patent Document 6 are known.
Patent Document 1, Japanese Patent Laid-Open No. 2003-187614;
Patent Document 2, Japanese Patent Laid-Open No. 2002-359090;
Patent Document 3, Japanese Patent Laid-Open No. 2003-317978;
Patent Document 4, Japanese Patent Laid-Open No. 2004-134146;
Patent Document 5, Japanese Patent Laid-Open No. 2004-134147; and
Patent Document 6, Japanese Patent Laid-Open No. 2005-206074.
The conventional LED lighting apparatuses have the foregoing configurations. Among them, the first conventional example (Patent Document 1) is characterized by preventing the effect due to variations in the forward voltage drops of the individual LEDs by connecting them in series, and by preventing variations in the amount of emission of the individual LEDs by equalizing the current flowing through the LEDs, thereby simplifying the circuit configuration by constructing the power supply from a single circuit.
On the other hand, LEDs, high-intensity LEDs in particular, have a high forward voltage drop of about 3 V. Accordingly, to light the plurality of LEDs connected in series, it is necessary to supply the LEDs connected in series with a voltage equal to or greater than the sum total of the forward voltage drops of the plurality of LEDs. To achieve this, the first conventional example has the boosting DC/DC converter consisting of a single circuit and boosts the power supply voltage. The boosting DC/DC converter utilizes flyback energy at energizing a coil, which repeats cycles of storing energy in the coil, first, and then discharging the energy. Thus, every cycle has timing at which the coil does not output any energy. To supply current continuously to prevent the LEDs from turning off even at the timing, the first conventional example has in parallel with the LEDs a smoothing capacitor for storing the energy once, and for discharging the energy at the timing at which the energy is not output.
Besides, the LED has forward voltage characteristics of nearly constant voltage characteristics. Thus, when the smoothing capacitor discharges its energy, the current flowing through the LED drops exponentially in accordance with the voltage drop of the smoothing capacitor due to the discharge. On the other hand, when charging the smoothing capacitor with energy, a slight increase in the voltage causes a current to flow through the constant voltage characteristic LED exponentially, thereby preventing a voltage increase of the capacitor. Thus, the capacitor cannot be charged with the energy sufficiently which is stored in accordance with (C×V×V)/2. Accordingly, to cause the current to flow through the LED stably, a large capacitance smoothing capacitor must be used to maintain the energy for a slight voltage change.
As described above, as for the configuration of connecting the smoothing capacitor in parallel with the LEDs having the constant voltage characteristics, although it is easy for the boosting circuit to make the terminal voltage to be applied to the LEDs constant, it is difficult to make the current flowing through the LEDs constant. A shortage of the capacitance of the smoothing capacitor will prevent the current from flowing constantly, and if the current flowing through the LED is intermittent, the light emitted by the LEDs is also intermittent, that is, turned on and off. Thus, the first conventional example has a problem in that the current flowing through the LEDs is intermittent, and hence the light emitted by the LEDs is also intermittent. In this case, if an object receiving the light is an object at rest, it is perceived quite normal even if it is illuminated by the blinking light source. However, if it is a moving object, it is perceived as if it were at rest for a moment because of a stroboscopic phenomenon, which is undesirable. In particular, when the LED lighting apparatus is applied to a lighting apparatus of vehicle lights such as headlights, the intermittent (blinking) lighting that causes the stroboscopic phenomenon is undesirable for driving the vehicle.
In addition, the configuration of the first conventional example, which requires a bulky capacitor as the smoothing capacitor with a large capacitance, has a problem of preventing the miniaturization of the apparatus.
As for the second conventional example (Patent Document 2), it includes, in addition to the boosting power supply, the constant current circuit for the individual LEDs. Accordingly, although the LED current does not become intermittent, when the plurality of LEDs are connected in series as in the first conventional example, the charge pump type boosting power supply of the second conventional example has the timing at which the energy is not supplied in every cycle as well. Thus, it has a problem of having a potential possibility of bringing about the stroboscopic phenomenon as in the first conventional example.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide, in the configuration of lighting a plurality of LEDs connected in series by using a boosting type DC/DC converter, a light-emitting diode lighting apparatus and a vehicle light lighting apparatus using the same capable of preventing the current flowing through the LEDs from becoming intermittent and the light emitted by the LEDs from becoming intermittent, and capable of reducing the size and price using a smoothing capacitor with small capacitance or without using a smoothing capacitor, thereby preventing a large-sized, large-capacitance smoothing capacitor from being used, and simplifying the circuit configuration.